I'm having trouble answering this:

The average size of a large fish that humans use for food has been decreasing over time, Exp-lain how, exactly, this happens - be sure to discuss genes, proteins, traits, and selection pressure.

This is all I can answer :(

The large fish are more desirable to catch, making their population decrease. The large fish will be forced to mate with the smaller fish, producing smaller offspring.

Any help is appreciated. This is a question from a practice test that I am taking for my bio class.